text:
What are the barriers to feasibility of stratospheric cruising for commercial air travel? (i.e. Concorde?)
In 2003, British Airways retired the Concorde for a few reasons.

Rising maintenance costs
Lack of demand due to safety concerns
Further decrease in demand due to 9/11 attacks

additionally, due to lack of competition, the Concorde was never properly maintained. Because no other airline had such a plane, British Airways kept the analog cockpit from 1970 up until the planes retirement.
Well, we live in a world where safety in aviation is continuing to increase, and the technology we have to utilize the world around us constantly gets better.
What engineering breakthroughs will allow commercial aviation to use the upper atmosphere and supersonic cruising speeds?
--EDIT--
I don't mind if you downvote my question, but it certainly annoys me if you downvote and you don't leave a comment telling my why. I've tried to make this question as objective as possible, if you're going to take the time to downvote, take the time to explain yourself, and I would be happy to try and mitigate your concerns.

Re "proper maintenance" and "keeping analogue flight decks". Production ceased in 1979. By 2003 there were 9 Concordes in operation. Has any airline or manufacturer ever retrofitted a new design of flight deck of existing 25 year-old commercial airliners? What would be the cost of designing and getting approval for a new flight deck for only nine aircraft? Isn't it only the military that repurpose old airframes in this way?

This seems to be almost entirely opinion-based.

@DavidRicherby while I think it's possible you could consider this as an opinion based question, I also think there are a lot of facts that influence the answer. I suppose I could reword the question to ask about the cost effectiveness of concord travel, if the costs are reducing, etc. Also when considering that safety is a factor, I think there is a lot more than just an opinion based answer here

@RedGrittyBrick hmm, seems that I have heard of civilian operators upgrading cockpits, but I may be wrong. Would make a good question though. ;-)

@flyingfisch I'm guessing that would be limited to aircraft still in production, where newer models have the upgraded cockpit, thus sharing the development/certification costs.

@flyingfisch: http://aviation.stackexchange.com/q/3789/1289 :-)

3 hour trip to London? Sounds like my daily commute not a transatlantic flight.

@Jamiec well you clearly don't live on the U.S. west coast, or maybe you do. Do you commute with an SR-71 blackbird? That would be fantastic

@BrianWheeler - No I live just outside London :) Did learn to fly in SoCal though!

I really think this question can only lead to broad speculation.  The number of possible reasons for or against is...large, to say the least.  So I've voted to close the question.  If you want to ask a questions about the particular issue you raise, that might be the better way forward.  Maybe use this as a place to find evidence to build a case for or against rather than asking that the case simply be decided in one go.

While this question might be a great topic for a blog post, or an editorial, or the Stack Exchange Aviation chat... it strikes me as way too open-ended for us to really answer concretely here.

@egid I've edited the question to diminish the possibility of opinion-based answers

Can we please get a blog post/editorial for this question? I'm really interested in the future of SST flight.

@BrianWheeler: I can't answer your question as it's been locked, but another reason I can think of is the risk of a window blowing out at high altitudes. The Concorde was designed to have small windows to reduce the risk of decompression if a window blew out at 60000 feet AMSL. So, if you have to go even higher, you would need smaller windows, or probably no windows at all to keep the plane safe.

Seems like a legit question. Granted its a complex intersection of technology, economics and regulation but then, so is all modern aircraft design.

There was some kind of competition, Tu-144.

I would argue that the Concorde was from the beginning economically unfeasible and instead served as a prestige technology for the British and French governments. Even the name Concorde, indicates that the two governments working together to produce the aircraft seemed the most important aspect of it.
Both British and France at the time had completely nationalized airlines and almost all their major aviation companies as well. Americans were going to the moon so Britain and France were looking around for a prestige technology of their own. Bigger and faster airplanes had been the rage for twenty years so the Concorde seemed like a cool thing to do. The Soviets came to the same conclusion.
In the US, civilian airline development was still wholly private so when when Boeing , Douglas et crunched the numbers they saw little value in supersonic transport with  it's non-linear cost. US supersonic transport never made it off the ground, but in Europe, the politically driven Concorde did because the people making the decisions weren't paying the bills.
As noted by others, the Concorde was essentially a military aircraft, produced by political compromise with the primary goal of expressing the technical powers of the governments involved. It was more an art project like building the Pyramids than it was a piece of mobile transportation infrastructure.
Edit: (May 14, 2014)
Some sources on the economics of the Concorde:
The Atlantic August 1977:

Already Britain, and France spent £1.46 billion \$2.3 billion to reach this  (and a London political economist has recently argued that
  the true cost is roughly  three times this amount)...
...In May 1976, Professor David Henderson, newly appointed professor of political  economy at University College, London, argued
  that the government's figure of  £1.46 billion shared between Britain
  and France was a drastic underestimate.  It had been reached by adding
  the yearly expenditure on the project at the current prices.  If these
  were adjusted to 1975 prices, and interest charges of 10 percent
  added, then  the cost of Concorde was not £1.46 bilion but £4.26
  billion (\$6.82 billion at the  present exchange rate of \$1.60).

That would be $22 billion US (2014 dollars.) Just to get the thing into the air.
They built 26 of the aircraft, 20 of which that actually flew paid flights. So, around 1.1 billion per plane, roughly the cost of a B2 stealth bomber. I can't find any hard numbers on operating cost but with 1.1 billion per plane sunk capitial cost, it's clear it would never operate at a total profit.
I believe the plane only flew for 20+ years because the British government essentially gave it away, eating the billions of in development and deployment cost and leaving the plane to just have to cover operating cost. I'm not clear that it even really did that.
Considering how much money was thrown at the problem, not particularly elegant or interesting.
Edit 02:
In rereading the parent, I realized I didn't make my answer explicit to the original question. He wanted to know what it would take to bring a Concorde like plane back.

My answer was really just saying, "the Concorde itself had no economic foundation so firstly, you'd have to find an economic niche for SST."
There was never an economic demand for the Concorde's speed of travel and there doesn't seem to be one now, either.
Technologically, you'd probably need something hyper radical like a nuclear powered air spike so the plane wouldn't burn tons of fuel and could fly over land masses without sonic booms or destroying smaller aircraft. (Seriously, it would have to be that advanced.)
In our current luddite era, it would be saner to think about inventing anti-gravity drives than think that the political regulators would let that type of radical technology fly, even if it existed.

Exactly right @TechZen.  It was a scaled up fighter. The US companies had already designed deep penetration supersonic bombers, and the XB-70 Valkyrie was actually built. So there was a good idea of the costs. Boeing designed the 2707 but it got cancelled because of economics.

@GdD got any citations for your 'scaled up fighter' claim?

"Concorde was essentially a military aircraft"?

@shortstheory - The engines and other tech for the Concorde and the other SSTs started out as military tech and where adapted to civilian. There were a spate of such conversion right after WWII eg the lancaster became the Lancasterian (http://en.wikipedia.org/wiki/Avro_Lancastrian) but by the late 50s, airliners where purely civilian with no military heritage. But there were no civilian supersonic aircraft so all the tech for the SSTs came straight out the military.

@shortstheory - It was also "military" in the sense of 1) being a pure creation of government instead of the market 2) being produced and operated first for political ends (look at the name) and having all the design faults caused by design by political committee.

@TechZen, I'm with you for the first two points, but what are the design faults on the Concorde? It performed remarkably well for a long time in service.

@shortstheory - I would argue rather emphatically that system, no matter how technologically nifty, which doesn't fulfill its core design goal has failed. In this case, the design goal was (notionally at least) to create an economically self-sustaining civilian airliner. I'm pretty sure that, while it did operate profitably some years, it never paid off its entire life cycle cost. Looked at another way, if all airliners had the life cycle cost pattern of the Concorde, we wouldn't have an airline industry at all. That's a design failure in my book.

@TechZen "It was also 'military' in the sense of..." Neither of those two things is a defining characteristic of what is military. Governments create all kinds of things; unless (roughly speaking) those things are designed for use in war, they are not military. Concorde was not designed for use in war.

Please edit your answer to read continuously, instead of having "Edit:" and "Edit 2:" at the end. Most people who read your answer will have come to it after your last edit, not before; they should be able to read a continuous piece of prose, not have to construct it from your "edit" instructions.

Concorde was economically feasible at the time of its design: you're not extracting much engine power to compress air, the mach 2 does that for you. At the time of design the high bypass engine had not been invented yet.

Concorde was a viable SST aircraft and was initially looking like it would sell quite well. Then US airports started banning it due to concerns over the sonic boom, which were really just sour grapes because US companies had failed to develop their own SST aircraft. That killed most of the orders.
It's worth noting that Russia flaw it's supersonic passenger jet for years quite successfully too. It had technical issues but, like with Concorde, the sonic boom issue turned out to be of little real concern.
A lot of research has gone into lessening sonic booms, but it isn't clear if this will actually help. Given the attitude of the US commercial flight industry it seems likely that unless Boeing's name is on the aircraft they would resist it anyway. I think it is more likely that such an aircraft would be developed and used mostly in the far east. China might do it, or Japan.

Haha, successful is a huge overstatement for the Tupelov Tu-144/Concordski. It barely made any revenue, had pathetic efficiency, was plagued by hundreds of technical issues in only 50ish flights, and pax couldn't stand the sound of its engines.

It's all about cost. Concorde stopped flying because it wasn't making money. The crash actually didn't mean that much economically, it's that it happened in the time of an economic downturn and executives couldn't be seen to get on a Concorde when they'd just laid off thousands of people. The airlines and airbus had had enough keeping it alive, so they are now all in museums.
Tickets were fantastically expensive. Concorde was basically a scaled up fighter jet with fighter jet engines. It used afterburner for some segments of the flight, which requires vast amounts of fuel.
Another reason SST failed is the sonic boom made it practically impossible to fly any land routes. They tried a few times but it was just unworkable.
Will it ever come back? I'm sure it will one day once they solve certain problems. You have the fuel consumption issue for one. There are engines that do "supercruise", giving high power without afterburners but they aren't exactly efficient. Either lowering fuel consumption or fuel costs is necessary. Second, you have the sonic boom. There are some promising designs out there which use specially shaped wings and fuselages to change and reduce sonic boom noise.
Personally I think that rather than supersonic travel things will jump to hypersonic, super-high altitude travel. Once you are in the really thin air you don't have to worry about sonic booms, and you have much less air resistance.
No matter how you cut it it's about cost though. If they can figure out a way to have supersonic travel with a cost enough people are willing to pay then they may build it, if not you'll never have it.

Minor point but Concorde did not use afterburner all the time. Only for takeoff and for acceleration from transonic to about Mach 1.7. It then "supercruised" on conventional thrust only.

@dvnrrs wow, the concorde could supercruise? That must have been some engineering feat.

@flyingfisch It definitely was an engineering marvel in many ways. This answer is right about the cost and other reasons but it sure was sad to see it retired.

"On average Concorde made [an] operating profit of £30-50 Million a year for British Airways in the boom years [...] British Airways reportedly received £1.75 Billion in revenue for Concorde services against an operating cost of around £1 Billion."  ... "the Airlines are not making back the money spent on the safety modifications and other upgrades. With some other big costs coming up, BA need to write off £84M now, rather than £150M in 3 or 4 years. Air France will write off a large sum of money too." - http://www.concordesst.com/retire/faq_r.html

The only way that their airlines made any money was that the airplane's initial costs were paid for by the UK and French governments. If the airlines had paid for the airplane they never would have broken even.

@GgD Very true. Also, they knew they were running the fleet into the ground (no pun intended). If they had to invest in new or overhauled airframes, avionics upgrades/replacements, etc. to keep the fleet going that "profit" would have disappeared. That wasn't an option anyway since the manufacturer shut down its production and maintenance supply chains.

There are many things factually wrong about this answer. As others have pointed out, Concorde did not use afterburners for the whole flight. Also, saying it was a scaled up fighter jet with fighter jet engines is misleading. It has a delta wing, which many fighters do, simply because that's the design which is efficient for supersonic flight, not because it was scaled up from something. Its engines were developed specifically for Concorde, but were partially based off of engines from a light bomber.

"When the air france one crashed it took a sizable portion of the customer base with it" is completely wrong as well. That flight was a charter full of German passengers on their way to a cruise. These were not their normal business customers who flew back and forth between Paris and New York or DC. According to British Airways, Septermber 11, 2001 had substantially more to do with why the Concorde was retired than the AF crash. Please, if you're going to speculate about something, at least get the facts straight. This answer needs some substantial editing.

The Rolls-Royce/SNECMA Olympus engines weren't part of any fighter jet I recall. Although SNECMA engines were popular on fighter jets at the time, so the Concorde's engines are probably some derivative.

@BretCopeland I don't interpret that as "the deaths aboard the crashed plane were a sizable fraction of the Concorde's market"; I interpret it as "when the Concorde crashed, the regular business customers stopped flying on it".

Economics.  Some innovations in engine technology that promise to lower fuel consumption for an SST can be applied to subsonic airliners keeping subsonic airliners ahead financially.
There's also the question of speed.  Aluminum is sufficient for an aircraft up to Mach 2.2.  Beyond that requires steel or titanium.
So, a plane with a max speed of Mach 2.2 is less than 3 times as fast as today's jetliners.  Counting taxiing, acceleration, and climb, this makes the SST's time advantage for the trip even less.
The SST will have a lower lift over drag ratio in all parts of the flight envelope compared to a subsonic airliner.  It will have greater parasite drag due to the greater speed.
If a plane with speed over Mach 2.2 is built, it will be heavier and therefore burn more fuel.  It seems there is no sweet spot for SST speed.  The sweet spot seems to be Mach .85 or hypersonic where the craft can exit the atmosphere.

Airframe heating is a big barrier to any airframe design and in consequence the fuselage needs to be able to grow longitudinally. The pressure hull needs to be encased inside a flexible, expandable sleeve-like skin. It requires the use of a great deal of heat resistant titanium and you can't use normal tools on the titanium.
Various issues could be overcome but you would need to develop a supersonic inlet with variable geometry surfaces to close and slow down air entering the engines at supersonic speed, or open to allow maximum airflow at subsonic speeds.
The development cost would be huge which is why I think the economics actually require a people mover rather than a VIP airframe with a limited market.
The market is definitely trans Pacific and trans Atlantic. Concorde never had the range for trans-Pacific. If you had the backing of say Singapore Airlines, Japan Airlines or a Big Chinese airline as a launch customer to operate trans Pacific then you would have some prospect of launching a new design.
For the Chinese in particular this would be a spectacular prestige product.

I agree that the golden market for a SST is trans-Pacific.  However, the passenger profile necessary to sustain a SST is the high end business traveler.  My company gags at the cost of Business Class on a transpac route right now.  They won't be putting the likes of me on a super-premium aircraft where the ticket cost is several multiples of the standard Business or First class ticket.  That's just the way it is.  So unless there is a technological revolution, or a new source of low cost fuel, the odds of success for a SST on the transpac route are low.

Could it happen? Yes. Anything could happen.
Is there a realistic chance of it happening at some point in the future? Yes, quite likely, if there's an incentive to develop the technology that would make it economical.

Is that moment likely to arrive during my lifetime? I seriously doubt it, especially given the lead times for such projects which are measured in decades, and I don't have centuries to live (which suggest of course that I don't envision the technology that makes such an aircraft economically feasible to become available within the next few decades).

There's no theoretical reason to assume that supersonic air transport can never be achieved. We know it's possible, it has been done. In fact it has been done several times.
Concorde, Konkordski, there was even (or so I've read) plans to create a transport version of the B-58 Hustler at some point, think a supersonic VIP and priority cargo transport for the US Air Force. There was an attempt to create a supersonic business jet by Sukhoi that was abandoned (I think) a few years ago. So we know it's possible, it's just not economical.
But who knows what may become economical in the future? If there's a large enough group of people who value their time more than the price of admission, there's a market. Right now that market isn't there, but it might emerge (the death of Concorde may have led to an increase in sales in long range business jet, if aviation laws get stricter, making it harder to use those, those owners might once again get interested in buying tickets on supersonic airliners and start asking airlines and governments, and aircraft manufacturers, to look into building them).

Or maybe there will be some revolution in aircraft design that makes it so cheap to build supersonic airliners that it just makes no sense to build anything else.

Of course that's all theoretical. There's no crystal ball anyone can look into and say definitely whether it will happen or not. But it might.

"during my lifetime", well how old are you? I'm 18, so that leaves a ton of possibility


text:
last 747 flight path, was this hand flown? How was this communicated with ATC?
Elal's last 747 flight made this nice path of a 747, how was this done?
Could this be entered into the computer and flown? or was this hand flown
The path looks quite neat
Also what is the ATC procedure for such a maneuver?

Related: How are special flight paths coordinated with ATC?

Thanks! It doesn't answer the first question

Possible duplicate of How are special flight paths coordinated with ATC? @foot 's answer to that question also answers "Could this be entered into the computer and flown?".

well, that's just sad. I took an ElAl 747 JFK-TLV back in the early 80s. Other than being a smoking flight, stuck in the very back row in seats that didn't recline, it was quite the experience.

FYI: The flight is still on flightaware: https://de.flightaware.com/live/flight/ELY1747

I don't believe anyone could hand fly something that precise.  A series of waypoints could be entered in the flight plan in a "connect the dot" manner, then the autopilot could be put in NAV mode to fly the pattern.  Nothing that difficult about it though, it would just take some time.
If they are at 10,000 feet this could be done under VFR, in which case no "ATC Procedure" is necessary.  I am pretty sure though, that the flight would have communicated their intentions with ATC.
I am curious about area just off the end of the pointed nose.  I am guessing that they flew past this point and made a reverse turn, effectively creating a loop in the depicted track that is cut off in the picture.  Do you have a full screen shot showing this?  Because if it isn't there, then this was either all photoshopped, or flown, then the loop edited out later.  A 747 can't just turn on a point...

The nose is the entry and exit point, so it came in on the upper side, flew the "left" side of the aircraft, around to the right side and exited again on the nose to the north. Here is a wider picture of it.

Thanks, that makes more sense, but why then is the aircraft symbol shown at the trailing edge of the right wing root as if it is completing the pattern there?

The screenshot is probably a play-back.

@MichaelHall You can see by the timeline at the bottom of the screenshot that this is a replay, the plane is just a little more than halfway through its flight.

"If they are at 10,000 feet this could be done under VFR, in which case no "ATC Procedure" is necessary" - Not really, you can be under VFR and still in controlled airspace thus requiring an ATC clearance for all movements.

Yes, I see now that it is a playback.  I'm not familiar with this format, seems odd that the full track is displayed before the aircraft symbol arrives.

@Ben, ok, I don't disagree, but would you care to put into an answer the ATC procedure the OP is asking about?  Perhaps:  "request vectors to outline the shape of my plane"?!

P.S.  I'm sure you know this seeing that you are a pilot, but for others reading this, just because you are in controlled airspace does not mean that ATC must clear your every move.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Can a drone hitting a plane be mistaken for a birdstrike?
Could a bird-sized consumer drone be digested by a jet engine, and not be recognized as a drone, mistaking it as a bird strike?
I'm thinking of the situation during the flight - like an engine making a short unusual noise, but works OK; not sure that can even happen eating a small bird?
As was pointed out, the debris of the object will look different in obvious ways when checking the engine after flight
I would assume a major factor is how the engine handles the drone, in terms of defects or running anomalies.
The battery comes to mind, hitting a turbine blade might be a problem - depending on the battery size.
There are certainly drones with very small batteries, that I assume will do no obvious harm, and certainly ones with batteries big enough to do harm for sure.

No.  Engine bird strikes leave a lot of evidence, as would a drone.  Post your edit - any drone going into a running engine is going to do damage.

Bird strikes leave bits of bird in the engine. Drone strikes wouldn't.

Good point - I was thinking of the situation during flight; Indeed there is a difference in what sticks to the exhaust edge - like lithium or liver ;)

@VolkerSiegel during flight all they know is "something is not right, land ASAP." They will probably not be asking "is it a bird, is it a plane", but may assume a bird strike unless they saw something different.

FYI, that "evidence" or "debris" from a bird strike is technically known as "snarge"  The etymology does not bear repeating...

@User58220 Nice - An, and that seems to be specifically in the airplane/ birdstrike context... Hard to resist to ask over at en.SE for the etymology ;) I'll do if anybody else indicates to be curious!

Would one be able to tell the difference before landing?  Not likely in many scenarios.
Case 1: Bird hits windshield: blood & guts are immediately visible.  Drone hits windshield, cracks are visible but not much else.  The crew CAN immediately figure out, if they've ever see a birdstrike on the windshield, that this was something else.  (Of course, it could just be a shattered outer pane -- those do fail sometimes, not necessarily due to any collision.  Never had one go in flight, so I don't know if the sound of the pane shattering might be mistaken for a collision {or vice-versa}, or not.)
Case 2: Bird hits near the cockpit but NOT on a windshield.  Sounds like a sharp crack, like a piece of gravel hitting your automobile windshield at highway speeds.  If a drone did the same, it would probably sound pretty similar, and without the blood/guts to see (or not see), the ability to notice what's missing may not be there.
Case 3: Bird hits the engine, and it starts running poorly. Sometimes you will smell the guts cooking in the core of the engine (i.e. they get into the bleed air system & thus into the air conditioning), and that's a clear sign that your engine ate a bird.  But the bird CAN miss the core and still cause problems with the fan blades.  Most birds that miss the core are chopped up by the fan blades & the crew had no idea until after landing that anything happened, but a big enough bird can cause damage that way.  So if the engine starts running rough, hot, and with some vibration, the absence of "cooked goose" odor isn't necessarily going to drive the conclusion that the collision had to be with something other-than-a-bird.  One might suspect that a lot of damage with no smell seems unlikely to be a birdstrike, but there are cases where some engine component has failed & caused engine damage (ranging from barely noticeable up to catastrophic), and you can't really know much about causality for the engine doing what it's doing until you're on the ground.
Now, all of this assumes that the crew doesn't see the object that they hit.  Which they very well might.  Everything in that case is obviously an entirely different discussion.

A small drone, like those commercially available, is not much bigger than your average raptor or even a goose, but the material it's made of is much harder. Not very many geese have aluminum shoulders affixed with stainless-steel screws and supporting carbon-fiber wing struts. As such, a drone will do much more damage to the turbine blades (or the windshield or any other leading surface). This is one of the big reasons for current restrictions on UAV operations; current rules treat them similarly to model aircraft, and that means a ceiling of 400 feet and a no-go zone five miles from any controlled airspace unless you have comms with the tower for the space you enter.

It's really dependant on 1) size and 2) type. I.E. if it's a DJI Phanatom, then it will just get shredded as its made out of plastic, possibly doing damage to the engine. On the other hand, if its a large multi rotor that's made of metal/carbon fiber, then the engine will most likely be damaged pretty badly.
So, TL,DR: if it's a consumer one, probably won't be much evidence, about as much as a bird, but with less blood. Of its large, then imagine a crane made of carbon fiber.

I'm trying to visualize the contact of a small Li-ion accumulator with a 45 k$ titanium single crystal turbine blade...

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is wind "CALM"?
What is the exact criteria for wind speed to be considered "CALM".
Different countries might have different values and i do not mind getting answers from those. But an answer from an ICAO annex would be prefered.

Wind "CALM" means a wind speed less than 0.5m/s (1 kt) according to ICAO Annex 3, Appendix 3:

4.1.5.2    In local routine and special reports and in METAR and SPECI:
...
d)  when a wind speed of less than 0.5m/s (1 kt) is reported, it shall be indicated as calm;

Before Amendment 75 of ICAO Annex 3 this read:

d)  when a wind speed of less than 2 km/h (1 kt) is reported, it shall be indicated as calm;

See also Annex 3:

Appendix 5, recommendation 1.2.1
Attachment C

The FAA, in their Order JO 7110.65X (Air Traffic Control), defines a limit of 3 knots.

2−6−3.  REPORTING WEATHER CONDITIONS
....
b. Describe  the  wind  as  calm  when  the  wind velocity is less than three knots.


text:
How does the target indicator on a jet fighter work?
How does the target indicator in Jet Fighter Aircraft work?  How does it stay on target? Is this queued from the radar, from infrared or is this image classification? What does the x in the target indicator mean? Are the other symbols around it part of the target indicator or related to something else?

Could you please point to the source of the image?

It could be any of them, depending on the configuration of the fighter. Some fighters acquire manually and tracks it with the missile's IR detector, mig-21 is such a example. Other more advanced fighters acquires and tracks the target with onboard radar or IR detector then let the missile lock on after launch. But sensor fusion is a thing now so you would expect the latest generation would collect data from all sources available and gets the job done.

There are two main questions here, mainly how does a fighter jet track a flying target, and questions about the symbols used on the Heads Up Display.
How does a fighter jet track targets
A fighter jet tracks its target(s) using its radar, which (as you are all aware) works by emitting EM waves and detecting the reflected waves bounded off by the target. The onboard computer processes this data and translates it to target information.
There are two modes of radar operation*. The first is a scan mode: the radar makes a sweep across a wide angle to look for targets. In this mode, the radar scope is wide, but target update rate is slow, and target information is coarse. The second is targeted mode: the radar focuses a narrow beam of energy at the target, which provides accurate readings with a high update rate. On the HUD, you can see "200Vc", meaning the jet is closing in on the target at a relative speed of 200 knots. Below that is "1000 FT", the distance to target.
* There are many modes actually, but to keep it simple, let's just focus on two.
A fighter jet does not identify a target optically nor using infrared. Neither of these two methods can provide accurate position information of the target. This may change in the future, but it is what the technology is now.
About the HUD
The HUD is from a F-18 (I am not sure which variant). Let's go over the symbols top to bottom and left to right:
Left column

On the upper left is a black square. It indicates that the trigger is pressed at this moment. It is displayed only on the recorded tape and is not visible to the pilot.
the boxed number is the indicated airspeed. It is reading 179 knots.
Going down is the angle of attack, or "alpha". It is reading 20.3 degrees.
Mach speed is 0.36.
G-loading is 1.7.
The max g-loading on this flight is 7.6 G.
Below that is the time in UTC.

Center column

On the upper center, the arrow pointing upwards "^" is the needle of the heading indicator. Above it is the ruler marked with heading numbers, which is apparently cropped off in this image.
The cross sign below "+" is gun cross. The cross is always displayed at the gun inclination angle as a backup.
The "w" sign is the attitude indicator. In the background we can see the numbers: the aircraft is pitched 26 degrees down and banked approximately 70 degrees to the right.
"GUN" indicates machine gun is the selected weapon. The cross indicate it cannot be fired, most likely because this is a practice mission and master-arm is not on.
"0" is the number of bullets left.
Near the bottom edge of the image, a bit to the right of center line, is the flight path indicator, a circle with three bars extending from its left, top and right respectively. It shows where the aircraft is travelling to, and it is useful for landing or flying low near terrain.

Right column

Current altitude is 15180 feet MSL.
Target closure rate is 200 knots.
Range to target is 1000 feet. If the range is longer than 1 nautical miles, it will display in nautical miles instead, up to the nearest 0.1 nautical mile (e.g. 2.3)

Symbol around the target

This particular circle is the target reticle for gun mode. A bigger circle is displayed if it is in missiles mode.
The center dot shows where the bullet will hit if the gun is pressed at this very moment. The circle moves around the HUD as the jet is maneuvered. For example, if the pilot pulls on the stick, the circle will move towards the bottom of the HUD because that is where the bullets are going.
Targets are identified by a square box on the HUD. The square is not displayed when the gun reticle overlaps with the target, which is the scenario in this image.
"IN LAR" means In Launch Acceptability Region. "IN LAR" is displayed instead of "SHOOT" because the target is designated as friendly. If the target is designated as hostile, "SHOOT" will be displayed instead.
Below the letters "LA", we can see a bold portion from 12'o clock to 1'o clock on the circle, and a mark at 1'o clock. This is the range to target. The mark rotates clockwise as target distance increases. For example, if the bold portion is instead from 12'o clock to 3'o clock, the target range will be 3000 feet.
There is a similar mark on the circle slightly pass 6'o clock, but the mark is outside the circle. This is the maximum range of the currently selected weapon. By comparing the bold portion of the circle to this mark, a pilot can tell whether the target is within range of the current weapon.
I have no idea what the "x" means.

@jjack I didn't say its a Navy HUD; it is the F-18 HUD. The F-18 is operated by many countries around the world, besides the US Navy.

@jjack because only the F-18 HUD looks like this. The HUD of every other military aircraft (that I am aware of) has a different layout. The F-16 for example has a speed tape and altitude tape; the F-15 has neither box nor arrow around the numbers etc.

"The center dot shows where the bullet will hit if the gun is pressed at this very moment." - At a predetermined range, right? Something like: the dot shows where a bullet fired right now will be when it is 1500 ft away (or some other distance that is fixed for the gun mode of the HUD), correct? I expect the "X" over the word gun either means to not fire the gun as IFF has determined the target is friendly, the gun is disabled, or the gun is out of ammo.

@ToddWilcox the cross over the word "GUN" means the weapon cannot be fired for any reason, such as master-arm is off, out of ammo, or some other config. It is not related to IFF; the F-18 has no locking mechanism preventing it from firing at a friendly target. About the dot: it works pretty much like the flight path indicator; its position on the HUD is adjusted such that it correlates to the outside world.

Yeah I didn't say the cross on the word "gun" meant the guns were locked out. It's just a HUD. Regarding what looks like an LCOS, I found the answer on my own, and the answer is "yes". According to this article (PDF), "...the pipper tell the pilot (for a specified range) where the round that was fired one time-of-flight ago is now." (emphasis mine) It just seemed to me that such a pipper is meaningless without a related range value to indicate where the round will be in three dimensions.

Oh, but the answer is also kind of "No", because the range is not predetermined. The LCOS uses the range reported by the radar to determine where to put the pipper. There are some flight assumptions involved in computing the lead, but theoretically if the pipper is dragged across the target while the gun is firing, a hit is highly likely. In practice it's not nearly that simple. I am more familiar with the EEGS funnel.

I think this is an F-22 HUD. I don't know this particular HUD, but I can make some educated guesses:

everything is radar based, a computer attempts to track any object in the radar's view
the circle means it is an acquired target, which means the computer is satisfied that it has tracked the target successfully and knows its position relative to ownship
IN LAR means "in launch acceptable range", that means that if a missile is launched, it might hit the target
the "x" probably means the currently selected target; in other words, if a missile is fired, it will target the circled object; it could also simply mean that the target is in range
the tick mark inside the circle is probably the radial velocity of the target; in this case the target is moving away from ownship at a moderate rate
the tick mark outside the circle is probably the relative aspect to target, but it might also be some kind of velocity indicator
the other notations have nothing to do with the target

No, I am pretty sure it is a Navy HUD and it is not an F-18, so that just leaves one possibility: F-22.

@jjack Nothing on this screen is cued by an optical sensor. The US uses three types of air-to-air missiles AIM-7, -9 and -120. All three are radar cued. The AIM-9 has an infrared seeker, but that is used only for homing, not target cuing.

@jjack That is used for ground targets. It cues bombs, not air-to-air missiles.

@jjack That is misleading. What they mean is "airborne" targets, like balloons, helicopters, and other slow moving stuff that can downed with guns. To a jet such targets are similar to other helpless ground targets. Optical pods are not used on enemy fighters.

Almost everything in this answer (except "IN LAR") is completely wrong.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How to execute a gear up landing on aircraft with low propeller?
On some GA aircraft such as the Piper Malibu, the propeller would touch the ground if the gear was up. How would you land the plane when gear actuators fail?
I would pull out the mixture knob and turn off magnetos, so the propeller is stopped when it touches the ground, but what is the official/best way to do this?
You can assume the propeller is constant-speed.

Unless you have a very long runway that you can cut the engine with plenty of altitude, yet still over the runway and still land and come to a stop on said runway, forget the engine/prop, put the aircraft on the centerline at the intended touchdown spot. You're in an emergency, you need to focus on flying the airplane not saving the engine or prop. You don't win by saving the engine while crashing short of or to the side of the runway.

@Farhan I removed the "avoid propeller damage" part in the title. I'd think propeller damage is just one of the concerns (the hazards caused by striking a propeller to the ground is another).

Well, you certainly aren't going to be able to go around once you touchdown.

Not only some, but in fact most GA propeller-driven aircraft have propellers that extend below the bottom of the fuselage. (The only exception that quickly comes to mind is the Cessna Skymaster; I'm sure there are plenty of others.)
Note that the aircraft will have some secondary method of gear extension, such as gravity drop, a manual crank, or a blow-down bottle. But there are still times where the gear cannot be extended and locked.
In this situation, you've got one emergency. Don't make it two emergencies by shutting down your engine. There's an aphorism - "If the gear fails, the insurance company just bought the airplane."1 Without gear, you're going to do some damage: scraped belly, probably tearing off some antennas. The aircraft will already require maintenance; the extra time and cost of an engine teardown is insignificant compared to the increased risk from a higher workload and a landing that the pilot may not have ever practiced before. (The approach with gear up and engine out will be different than that with gear down and engine at idle in a simulated engine failure.) Don't risk your life to save the insurance company money.
Additionally, just stopping the engine isn't sufficient to prevent a teardown. You must also rotate the prop so that it's horizontal, which requires slowing substantially as you pull the mixture to stop it from windmilling, and then using the starter to move the prop.2 You'd need to prevent any substantial contact between the propeller and runway, as Lycoming's Service Bulletin on propstrikes that require inspection and repair (Continental's is similar) includes the situation of

Any incident, whether or not the engine is operating, where repair of the propeller is necessary

After all of that, there are some best practices:

Declare an emergency. ATC will help you, have equipment ready on the ground, and clear traffic around you - but they need to know about it first.
Use your checklist. An airplane with retractable gear will have an entry for gear failure; follow it. In particular, you'll shut down the electrical system before landing, and probably brace open a door for exit.
Land on a paved surface. Dirt or grass may seem softer, but they aren't as smooth. According to an AOPA flight-training newsletter, "Statistics suggest that putting the airplane on the asphalt is likely to cause less damage to you and your airplane than putting it on the grass."

(1) I've also heard "The insurance company buys the aircraft when you take off; when you land safely, you buy it back."
(2) Or you could do it in the flare - a suggestion so dangerous I include it only for completeness.

I completely agree with @NathanG's advice, shutting your engine down makes a bad situation worse and limits your options. It also upsets attitude and trim, creating distractions when you least need them!

I can't stress how important it is to remove all power sources in the cockpit when you make a gear up landing. A powered engine with fuel flowing the pipes spinning the empty propeller shaft while you slide down the runway with the belly of the aircraft?

@GdD: if its' limiting your options, then you're cutting the engine too early.

It's never too late to have options @kayson. Say you get a gust of wind that blows you way off centerline and into the path of an obstruction, or a fire truck pulls onto the runway by accident.

You're about 20 feet above the runway when you do that. Yea a rabbit can suddenly jump onto the runway too. But why? I've known people who died due to post-crash fire. I've also seen other pilots cut the master then survive. My choice? By 20 feet, I'm committed to land. I'd rather risk skiing to the grass due to crosswind than a post-crash fire.

@kevin I wonder how much fire risk is actually reduced by turning off the power. I would think that much more of the ignition risk comes from a piece of aluminum scraping across a piece of asphalt at 100 mph than from the engine running.

Don't shut down your engine, it's a bad idea. Your aircraft will require substantial repairs in any case, the most important thing is to land safely. Loss of engine power is a serious emergency, worse than landing gear not lowering, cutting your engine is multiplying the risk several times over. Survival in this case depends on a smooth, controlled landing; cutting the engine causes risks to this:

Higher descent rate: most aircraft have a higher descent rate without engine power than at idle, sometimes this is substantial. A higher descent rate is not good when the smoothness of your landing is critical
No control over descent rate: without your engine you cannot add a burst of power to smooth out a sudden drop. This could lead to you slamming down on your belly instead of having a controlled landing. It could also lead to landing short of the runway if you cut it too soon. This has happened several times, and often is not survivable
Inexperience with dead stick landings: Few people have ever experienced how an aircraft without power handles and lands, you'll be entering new territory for which you've had no training
Less control without the engine: Airflow (at least in single engines) contributes to elevator and rudder authority, even when at idle, you need all the authority you can get for the most controlled landing possible. Also, you can use your prop to keep you in place in a crosswind
No option to go around: once you cut your engine you are committed and have no option but to land, what if a big gust blasts you off centerline and into the path of a hangar? Or what if you aren't perfect and want to have another try?
Change of attitude and balance: Cutting the engine will give some pitch down on most aircraft which would need to be controlled for, but you've cut your master switch so your electric trim is out - better start winding that lever or have the arms of a gorilla
Distraction at a critical time: Landing is one of the busiest times in an aircraft, you have to watch your airspeed, descent rate, and if it's bumpy you'll be sawing away at the controls. Having to pull the mixture - bump the prop to it's horizontal (no point cutting the engine otherwise) - cut the mags - fuel stopcock off - master switch off all while arresting an increased sink rate, trimming, and coming to terms with all the balance changes will all add massively to your workload when you most need to be concentrating on making a controlled, gentle descent.

A good landing comes from a good approach, cutting your engine throws that away, and you cannot go around if you hose it up or conditions aren't right. Instead make a controlled approach, brief your passengers on emergency procedures, tell them to brace. When your airplane comes into contact with the ground use whatever control authority you have to keep it controlled, then once it is slowed cut your fuel, mags, etc. As Bob Hoover said "fly it as far into the accident as possible."

Your instinctive actions are correct; killing the engine is the correct way to handle this emergency.
Here's part of the (generalized) procedure for landing a propeller aircraft with the gear up:

Pull the circuit breaker to the gear. We don't need the gear suddenly popping down and surprise us. Once you've made up the mind that the gear won't go down, stick to that decision.
On short final, door latch open. The last thing you want is trapped in a smoking or burning aircraft.
On crossing runway threshold, mixture cutoff, ignitions off, master switch off
Flare as much as possible and aim to hit the ground at a slower speed
Expect smoke to fill the cabin due to heat caused by friction, or even a fire
If you have access to a fire extinguisher while inside, extinguish cabin fires only if absolutely necessary.
Get out of the aircraft ASAP.

Unless the aircraft has props hoisted on high wings, it's almost certain that the prop will hit the ground and smash. Even if the prop is not rotating as it hits the ground, it'll still be chopped off. The reason to turn everything off is to minimize potential sparks, fire and explosion as much as possible. Also expect extensive damage to the aircraft.
Obviously you'd want pretty little red lights standing by as you make the approach.
As for other comments saying you should not turn off the engine, obviously no sane pilot will do this while at 5000ft and 13mn from the airport, right?

Metal propellers are made of aluminum, so they will not spark. Also, if you decide to land gear up in something like a Malibu with gravity extend gear, pulling the breaker will cause the gear to extend.

The ignition systems of the engine spark during operation. Not to mention the chance of a fuel tank rupture is very real. There is no guarantee that the gear will extend - the assumption here is that we have done everything and it just won't come down.

This is very wrong! There is no recommended practice to cut off your mixture and shut down the engine - you are simply adding one emergency to another and giving yourself no option other than land. People have died following this advice!

Nope, not cutting it is very wrong! Passengers have perished in the resulting fire when a twin prop made a powered gear up landing in the field. If you don't do it you're simply inviting disaster. In fact it's a standard practice of any crash landing.

exactly like this.

(I will note that I am NOT a pilot, only an enthusiast.)
Kevin's answer appears to be the most correct one, without giving the actual CORRECT answer. The only correct answer is to perform whatever emergency procedures are contained within the POH, or Pilot's Operating Handbook for the aircraft you are flying, coupled with any revisions, etc.
(Please note, the following links are to scanned in copies of the POH's, and are not links to the manufacturers websites, but to private companies)
I did some quick research, and three aircraft, specifically the Piper Arrow PA-28R-201, the Cessna 210M Centurion, and the Cessna 421C Golden Eagle, all have emergency gear up procedures. In all three cases the procedure includes ignition off, magneto's off and fuel off.
There may very well be an aircraft whose POH indicates full power and fuel during emergency gear up landings. However, the only way for that to be the correct procedure is for some documentation to say it is.
Also, no where am I suggesting that during an emergency you pull out the POH and read it. Very contrary, I am suggesting that you read and re-read the applicable documentation for the aircraft you are flying at the moment until you are very familiar with it.

I agree that following the POH is the correct procedure. However, the Cessna documents call for (1) master off; (2) touchdown; (3) mixture/mags/fuel off - in that order. The Arrow does not have a gear-up landing procedure; it does discuss landing gear-up and call for electrical and fuel shutoff before touchdown - but only in the checklist where the engine has already failed.


text:
On which part of the descent does a pilot begin to flare?
On which part of the landing process does the pilot begin to flare?

Flare height is typically 10 ~ 30 feet, which varies with airplane size. At this point, the aircraft has already crossed the runway threshold and is above the runway. Usually, you will smoothly reduce throttle to idle as you cross the threshold (if you don't, the aircraft might climb away!), then flare. That's around 3 to 10 seconds before touchdown.
In a small GA aircraft, typically you flare when you're around 1 to 2 stories high. The speed will bleed off as you flare, the stall warning might sound ...... which is alright, because you would only stall 2 feet.
On a large airliner, flare happens a bit higher. The cockpit of a 747 or A380 is also much higher above the ground (imagine stacking two cargo containers on a trailer, then drive this on a road), which means flare happens much higher from the pilot's perspective.
Learning how to flare is one of the things which cannot be practiced in a simulator, because we need peripheral vision to judge height. I have heard seasoned airliner pilots having a hard time landing small aircraft, because they would always flare too high.

This answer is a bit dated since it applies to 747-100 and -200 aircraft in the 1990s. However, the same principles still apply, and I'm sure the same technique would work now on current 747s. Whether or not anybody uses this protocol now I do not know.
We flared the aircraft by the numbers without reference to peripheral vision. The use of outside vision was, of course, critical for runway alignment and any necessary crosswind corrections.
We flared with reference to the radar altimeter. If it was a 'talking' altimeter, it would announce the altitude of the main landing gear above the ground in the following increments: 100, 50, 40, 30, 20, 10, with the increments being in feet. If the the radar altimeter was an old non-talker, the flight engineer would read those increments.
Coming down the typical 3 degree glideslope on a stabilized approach, the nose would be 5 degrees up, and the fuel flow would be about 5,000 lbs per engine. When you heard the 100 foot call, that served simply as an alert. When you heard the 50 foot, you pulled most of the power and raised the nose 2 more degrees. When you heard the 10 foot call, you pulled the rest of the power while raising the nose an additional one degree, maybe a little more if you had the runway length to try for a real greaser at the risk of floating. Then you just waited. Touch down of the main gear would occur within 1 to 3 seconds. If it didn't occur within that time frame, you were floating and would want to reduce your pitch a degree or so.
These 'numbers' would be adjusted if you were making a non-stabilized approach, which we often did back then when conditions permitted because stabilized approaches used more time and fuel. Plus, a non-stabilized approach is more challenging and thus much more fun. As I understand it, they're frowned upon these days even to the point of ops specs requiring a stabilized approach.

Here is a nice video that describes it well for a small airplane, worth a watch.
The flare is a key part of the landing and flaring too high can cause the plane to fall and possibly bounce.
A late flare or no flare can cause the plane to come in flat which can lead to a variety of issue that may or may not include bouncing on the nose wheel and other possibly dangerous situations.


text:
What is the use of saw-teeth present on the nacelle outlets of modern turbofans?
I've noticed that there are saw-teeth on modern jet engines such as the GEnx and the Rolls Royce Trent 1000. What is their purpose?

You mean on the back?

@fooot, the question that is said to be similar, is addressing a more specific issue.

Can you edit the question to explain how this one is less specific? Right now they are both just asking "what is their purpose," and the answers are very similar.

These "saw-teeth" or "chevrons" or even "acoustic liners", are there to help reduce noise generated from the operation of the engine.
As hot air from the inner core of the engine mixes with the other, cooler air, it generates noise. These "saw-teeth" are there to help smooth that mixing, thereby generating less noise which is the result of turbulence. These are seen on the Boeing 747-8 (as seen in your picture) and the Boeing 787.
The reduction of sound generated means that less noise insulation is needed for use in the fuselage of the airplane. Less insulation means less weight. Less-weight equals less fuel burn, which means that chevrons are great for passengers, and economics.
There's only one difference from your picture. As much as we would like them to blow kisses to everyone, that's not what they are there for. :)

Thank you so much @Zizouz212, the answer was very informative and helpful.

To quote from the Wikipedia article on the subject:

In aerospace industry, chevrons are the sawtooth patterns on the
  trailing edges of some jet engine nozzles[1] that are used for noise
  reduction. Their principle of operation is that, as hot air from the
  engine core mixes with cooler air blowing through the engine fan, the
  shaped edges serve to smooth the mixing, which reduces noise-creating
  turbulence.[1] Chevrons were developed with the help of NASA.[1][2]
  Some notable examples of such engines include GEnx and Rolls-Royce
  Trent 1000.

Banke, Jim (2012-12-13). "NASA Helps Create a More Silent Night". NASA. Retrieved January 12, 2013.
Zaman, K.B.M.Q.; Bridges, J. E.; Huff, D. L. (17–21 December 2010). "Evolution from 'Tabs' to 'Chevron Technology’–a Review" (PDF-1.34 MB). Proceedings of the 13th Asian Congress of Fluid Mechanics 17–21 December 2010, Dhaka, Bangladesh (NASA Glenn Research Center. Cleveland, Ohio). Retrieved January 29, 2013.

Please link to the article, don't just quote it and paste the citations as if this was your own content.

@egid If its a quote, it isn't my own content is it? Link only answers are discouraged. Also, pasting saves the trouble of going to the link. Also, in some cases the relevant text is buried on the page, so I am extracting the relevant part of the page.

I think it's safe to say that quote-only answers are also discouraged, but maybe we should start a meta thread about it. Alternately, maybe answers like these should be community wikis. Either way — what if someone wanted to read more? There's no reason to put a fair bit of effort into including the citations and then not drop in the link to the article.

And, in this case, you actually pasted the entire article

@egid Normally I would never do a copy and paste answer, but in this case the wiki completely answers the question. The question in fact in a lot of forums would not be allowed because it is trivially answered by the wikipedia.


text:
Why don't modern aircraft have landing gear that can swivel to compensate for crosswinds?
When landing, crosswinds can be very expensive. They cause go-arounds, diversions to other airfields and even crashes.
Currently pilots crab in the air to counteract crosswinds and, at the last moment, throw the aircraft around to face directly along the runway. This is a challenging and risky manoeuvre - it can cause asymmetrical stalls and worse.
The nose wheel can already be rotated. If the main gear could also rotate, the plane could land in a crabbed attitude and continue that way along the runway until speed had dropped to taxiing speed.
How come aircraft don't have this feature?
EDIT
It has been flagged that this question is similar to a previous one, Is the main landing gear of a large airplane designed for sideways (crabbed) touchdowns?
My question is different. In those cases the landing gear does not actively turn to face in the direction of travel relative to the ground, it is merely designed to cope with the extra stress of hitting the runway obliquely. The allowed angle may be quite small. A swivelling undercarriage would allow for a large angle of crab with no attendant stress to the gear because the wheels would be aligned with the runway.
As pointed out on that other question, this design feature was adopted for the B-52. I am asking what factors persuaded designers to abandon this idea.
Videos (2) of B-52 landing in this manner
http://www.skilledpilots.com/video-this-is-how-a-b-52h-lands-in-a-crosswind-and-stays-completely-sideways-while-deploying-a-parachute-and-slowing-down-on-the-runway/

This is a risky manoeuvre - it can cause asymmetrical stalls and worse.  Do you have some reference for this? How would you intend to deal with the asymmetric thrust?  You seem to be looking for a solution (an expensive, complicated, more things to go wrong and more cost to the airline solution) to a problem that doesn't exist.  It also would not address go-arounds and diversions.

possible duplicate of Is the main landing gear of a large airplane designed for sideways (crabbed) touchdowns?

This seems not a complete duplicate, as that another question is more about "can it handle without breaking". XB-52 Stratofortress  has the swiveling gear. Indeed, why such design is uncommon?

I agree with the OP, this is a bit different than the other question and I think it could use it's own answer.

Also, for the OP, I don't know if "abondon" is the right word.  I think the better question is why they decided it's not cost efficient for civilian commercial aviation.  I'm sure they'd still use it if it was cost effective but (for some reason I'm hoping someone will describe in detail) it must not be.

Just as a matter of information, in 2 years on 727s and then 10 years on 747s, I never had to divert or go-around because of a crosswind. The 747 in particular handles crosswinds quite well.

Fascinating question.  Chasly, it does seem to be that the problem you outline "just doesn't exist".  Could it be you've overstated the putative problem in question?

There is one aircraft I know of that can do exactly what you state, the B-52:

This plane has steerable dual-bicycle gear which allows the crew to point the gear along the runway while the fuselage is "crabbed" up to 20* off the runway centerline. AFAIK the gear are slaved to the ILS; you dial in the OBI to a course and the gear will point that way even if the aircraft points differently.
The reason this was done is primarily due to the design of the rest of the aircraft. It's a very "dense" design, cramming a lot of weight into a very small package, requiring massive wings and tail just to keep the thing under control at altitude. These surfaces then become huge sails at lower altitudes, and with the high, anhedraled wing arrangement on a relatively low fueselage, there's not much room for error. B-52 pilots often say you have to fly 2 seconds ahead of the plane, it's that slow to react to pilot input. So, classic "de-crabbing" techniques involving a hard rudder with counter-aileron just aren't going to work with a BUFF. You have to land crabbed and correct after touchdown, and on a dry runway that isn't a good idea either with an aircraft of this size.
Modern airliners, for their size, are much easier to maneuver on the whole than the BUFF. Really big ones, like the 747, actually have steerable main gear as well, but AFAIK the 747's mains can only be actively steered on the ground, and because they counter-steer to the nose gear (helps bring the tail around corners; imagine driving something about 4 times longer than an 18-wheeler) they would actually be pointed further off the runway line when de-crabbing.
Four- and six-wheel main gear systems used on larger airliners (777, 787, A380) actually have a small amount of castering built in; if you watch YouTubes of crosswind landings (there are several) you'll see that the rear two wheels of each gear touch first and actually twist the entire "truck" into line with the aircraft's actual direction of travel. Then as the plane settles on the gear and the fuselage lines back up with the runway, they'll straighten. Planes with one- and two-wheel main gear don't need this as much (and such a system isn't as effective as the first point of contact of a tire is directly level with the gear strut instead of behind it).

Thank you - When you talk about  passive steering, do you mean what would normally be called castering or is there something more complicated going on?  Do you think in principle it would be very difficult to modify a 747's counter-steering to emulate the B-52 - if so how come they didn't - is it just down to plane length being too wide for the runway when traveling diagonally? How come planes with passive steering need to straighten so soon?

Yes, I'm referring to castering. as far as the 747, I'm sure some enterprising engineer could rig the gear to face the runway. However, runways for commercial airliners tend to be very wide, several times the span of the gear. That is in part due to the need to crab/de-crab on landing; even in areas where the prevailing wind direction is fairly regular, it's rarely directly in line with the runway.

B52 Crosswind Landing Video

I point out that the B-52's wing tip mounted landing gear would provide a force tending to induce ground loop if the plane were landed in a wing low forward slip (as it normal).

Another video: https://youtu.be/9tyc9Imjaqs

Not helping matters is that the B-52's rudder is tiny for an aircraft of its size, especially compared to that on something like a 747.

My landing gear engineering experience is with jet transport aircraft other than the B-52 or 747, but based on general principals of the discipline here are a few comments related to points which posters have raised.

The unusual layout of the B-52 gear -- essentially four main gear nearly in line on the fuselage and no nose gear -- is better suited for a gear-rotating system than the typical tricycle-gear airliner.  Designers always have to be concerned with failure modes, and if a failure were to cause just one main gear truck of a tricycle layout to rotate, or either one suffered a hardover failure, the resulting imbalances could be disastrous: A big yaw moment immediately following touchdown; asymmetrical braking forces; lack of braking from one truck if its tires  fail; perhaps a resonant strut vibration stressing it beyond its design limit. The list goes on.
Payload would be decreased by a least the equivalent of a few passengers.  No one in "management" would be good with that, for a system which has no real demonstrated need (see for example the poster's comment "in [12 years], I never had to divert or go-around because of a crosswind").  In my personal experience attending engineering meetings, there's pushback if someone wants to add say a 50 lb item to a 500,000 lb TOW aircraft. Figure that a steerable main gear plus associated systems could easily weigh ten times that, and it becomes a complete non-starter.  A big dent in the aircraft's profitability plus introduction of failure modes which could lead to hull loss, all to to solve something which is closer to an annoyance than a real problem.
Slight aside: Pre-rotation of main gear tires is in the same "ain't gonna happen" category, for similar reasons (I was the person at my company responsible for sending polite replies to people who contacted us with the suggestion).  A lot of weight/complexity, and more failure modes than lay people could imagine, for something which turns out to not really be a problem: all that tire smoke doesn't really amount to much in terms of tread wear... at least not enough to make pre-rotation anywhere close to worth it.
Regarding a poster's comment about steerable main gear on the 747 helping with turns during taxi: A related reason why the "inner" (body) main gear trucks on the 747 are steerable is to reduce the stress within those tires caused by something  called "scrub". Scrub angle is the difference between which way the aircraft is pointed and which way a particular tire is pointed.  That is, at any particular instant a tire will generally not be on the same effective turn radius as the aircraft. The tire "twists" as a result, putting more stress into an already highly-loaded tire (they may not be metal, but tires are as closely designed to loads as any other element of aircraft structure). So even if the aircraft could make a tight turn without the main gear steering, doing so would put an unacceptable load into the tire.

Geez man, why bother answering unless you have some real experience with the issue at hand?  :) Damn, this has to be one of the best answers on the whole site, thanks for the incredible read!!

Isn't the steerable main gear on the 747 creating the same disadvantages as you mention in your first two points (failure modes, payload reduction)?

I think one aspect one might consider in every refinement one adds is the risk of malfunction. In this case, what's the risk that a swiveling  happens when you don't want it to. I suspect that the conventional crabbing procedure might not be so bad once you factor in the additional cost & complexity of a swivel & the risk that the gear swivels when not commanded or swivels not commensurate with the amount needed.

Goodyear did produce a crosswind gear system for general aviation aircraft in the 50s. It was a factory option on some Cessna taildraggers and Beech Bonanza tricycle gear aircraft. The axle is spring loaded to rest in a cradle until the airplane touches down in a crab, at which time it pops in the direction of the plane's momentum. Rather weak and finicky brakes gave it a bad reputation, as well as pilots being nervous about touching down in a crab.
Boeing 737s apparently have a more modern version in the main gear struts, as they can be observed taxiing while in a crab by aircraft behind them.


text:
Can an aircraft be docked at the gate from the right seat?
Not all aircraft can be docked from the right seat, because they have a wheel tiller only on the captain's side, but some can (e.g. the B747-400 of the linked question).
The usual docking system at gates looks like this:

(source)
It seems the azimuth guidance is based on parallax, and is designed for the left seat.
What happens if the pilot in charge of docking the aircraft is seated on the right side? How is the alignment performed without the support of the visual docking system?
Related: Are large passenger aircraft steered during taxi by changing the angle of the front landing gear?

The system you showed may actually be the type that does not depend on the position on the pilot. Edit: The version you have works with a Moiré pattern, and is indeed dependent on which pilot is looking at it.
I dug into the AIS Netherlands information for our country's biggest airport Schiphol (EHAM/AMS). They have a few different systems; the one you link seems to be a lot like the SAFE DOCK system, which uses a Laser Range Finder (LRF) system to identify the position of the aircraft

Due to the digital display presentation, both pilots get the correct alignment information as well as the closing-rate and stop information.

I'm guessing that this system will become more common to allow for more flexible crew tasks during taxi. Furthermore, this system can detect obstacles, and as such provides extra safety.
The two other systems on EHAM do depend on the seating of the pilot. The effect of the wrong pilot parking the plane is very simple: your plane will be parked off-center with a deviation exactly equal to the spacing between the pilots: your plane will end up such that the co-pilot is sitting where the captain should be. The fact that the AIS does not quote any procedures for this says it all: you just shouldn't do it and wake the captain for a minute to do his duties.
Of course, there are procedures for when the system is not available (emphasis mine):

The system is operated by an employee of a handling company, who also keeps a safety watch during the docking. The pilot of an arriving aircraft has to be sure that the system is activated. If not, pilots should not enter the aircraft stand and stop before the red boundary line, until the system is activated or a marshaller has signalled clearance to proceed.

The logical conclusion we can draw here is that, if the systems are not available (or, in the case of the question, if the captain is fast asleep), a marshaller will guide the plane in - either way, a marshaller will stand by to oversee the docking and make sure the SAFE DOCK system is living up to its name.
As @reirab helpfully pointed out, the marshaller's job may be nothing more than a simple clearance to enter the gate and let the pilots do their job; they should be perfectly capable of parking an aircraft without the displays. In the aviation world, it's perfectly acceptable to spend a lot of money on these things if they can increase safety; if only due to reduced workload for the pilots.

+1 for wake the captain... to do his duties alone! :)  Sadly, your linked document has expired.

@FreeMan Since this will periodically be the case (only the latest version of the AIP is available, and the release date is hardcoded in the URL), I changed it to a version found on archive.org. Thanks for the heads-up.


text:
What is the story of this aircraft?
From this page about nuclear bombers:

It seems out of Wild Wild West. What is this (Russian?) aircraft from another era? and what has been its use?

Update:We know now the image has been created using CG software, based on a real aircraft, that seems interesting and is by itself quite a mystery and technical feat that deserve explanations.
Why this design? gondolas, size, holes in wings, angular panels...? for apparently not so many people on-board.
Images of actual aircraft are found in answers. Also this one:

Source.
Thanks to @user52723a and @RonBeyer for providing details.

If you look close, you can see the people are computer generated... Here is an article on Snopes.com, not that Snopes is a great reference, but the computer-generated-ness is a lot more apparent.

@RonBeyer That's not "an article on Snopes" -- it's a post on Snopes's discussion forums. (But I agree that it's more obvious there that the images are CGI.)

@DavidRicherby Article, post, rant, not a lot of difference on the 'net these days...

@RonBeyer No, there's a huge difference. The articles on Snopes are reasonably well researched and are OK as sources; the discussion forums are just random people posting random stuff. You wouldn't, I hope, say "According to an article on some respected news organization's website" if what you actually meant was "According to the comments posted under that article." Same thing here.

It's also similar to the ANT-20 and other aircrafts from that era

@DavidRicherby Agreed. There's a huge difference between "a lecture" and "someone belching in the audience".

It is in fact fake but its most likely loosely based on a real plane. The Russian Kalinin K-7 contains a lot of the same design elements.

(source)
It appears that the real plane was also boarded from the landing gear area.

(source)
On a bit of a funny note this article mis-uses the photo you have linked in reference to the Kalinin K-7

Interestingly, the picture in the article is not exactly the same as in the question. The one in the article does not have this "cage" over the cockpit, but has additional cannons. A strange case of manipulation, IMHO ...

It's a fake -- somebody having fun with computer graphics. Never existed in real life, sorry.

That picture has been around the internet for several years, but it's pure vaporware.  The thing would be way larger than the Spruce Goose if it were real, and probably even more badly under-powered.  All CGI, nothing more.

It is supposedly based on the Kalinin K-7, but much bigger

@user52723a: Thanks for providing this clue, good find.


text:
Is this aerodynamic braking procedure normal in a 747?
I found this instagram video via YouTube:

The description in the YouTube video states that:

This aerodynamic baking procedure is applied to reduce wheel brake and engine wear

I have two questions:

Is this a normal procedure for the 747? If so, does anyone have an excerpt from the pilot manual for this?
How would this reduce engine wear? As you can hear in the audio of the video, the thrust reversers seem to be applied the same.

Since that plane's body in white with no (visible) tail number, it may be testing. With no main deck windows, it's definitely a freighter.

Oh, the nose wheel is still in the air. So the pilot keeps applying elevator during the whole landing. Was wondering what this “aerodynamic braking procedure” was supposed to be. Hard to see the nose wheel in the shot.

I can think of one, and only one, circumstance in which this would be useful: landing with wheelbrakes completely inoperative.

"Hard to see the nose wheel in the shot" --@Michael. Yeah, real hard! Is the NLG actually down? I don't even see gear doors open. There's a bright spot right about where the NLG should be, but that could be a reflection from the structure behind the plane.

@FreeMan That bright spot is the taxi or runway turnoff light on the nose gear structure. If you watch the linked YouTube video, you'll clearly see the gear is down.

I thought it might be a NLG mounted light, but wasn't sure - I thought the landing lights were mounted with the MLG, and didn't know about a taxi/turnoff light. I see the tire, now, but didn't when I was looking a bit ago. Thanks, @Bianfable

What's "this aerodynamic baking[sic] procedure"?  Your question needs to be complete in itself without access to the off-site links.  (Context in the answers suggest that it's referring to holding the nose high for some time, but that's not mentioned anywhere in the question).

This is not a recommended procedure for landing the 747 (or any other jet airliner I know of). The FCTM (Flight Crew Training Manual) says this:

After main gear touchdown, initiate the landing roll procedure. If the speedbrakes
do not extend automatically move the speedbrake lever to the UP position without
delay. Fly the nose wheels smoothly onto the runway without delay. Control
column movement forward of neutral should not be required. Do not attempt to
hold the nose wheels off the runway. Holding the nose up after touchdown for
aerodynamic braking is not an effective braking technique and may result in high
nose gear sink rates upon brake application.

(Boeing 747-400 FCTM - 6.17 - Landing Roll, emphasis mine)

Same warning for A320/A330 + risk of tail strike or hard nose gear touch down when coupled with autobrake. Using this technique may indicate a former military pilot, who used it on fighters due to small brakes.

@mins Good point about fighters (related question: Is aerodynamic braking really effective?)

@mins, not sure how accurate this is, but I was once told that you can tell what branch of the military a pilot came from based on how they land. Navy pilots will land tail-down because they were trained to land on aircraft carriers and had to catch an arrester cable with the tail hook.

@SethR All planes land tail-down, though.  Find me a pilot who puts the nosegear down first!  The big difference with carrier landings is that you hit the deck at like -700fpm, and that's a habit that doesn't really carry over well to heavy airliners.

The high nose gear sink rates is perhaps a polite way of saying "now that you've finally stalled both the wings and the tail, anything that was up in the air will just slam into the runway as-if in freefall but if you brake, it'll do it even faster than that". The brake application is what you do to achieve said stall first, and second it generates an additional "slam torque" :) It's very much an uncontrolled situation at this point, since controls are ineffective.

@J...: "Find me a pilot who puts the nosegear down first!"  Here ya go.

@Vikki Never say never, I guess.  That's bonkers.  Like trying to land a unicycle.

@Kubahasn'tforgottenMonica Neither the wings nor tail(plane)/horizontal stabilizer are fully stalled on a normal, properly-executed landing, and if the tailplane was, it would be impossible to hold the nosewheel off the ground. Furthermore, "the brake application is what you do to achieve said stall first" makes no sense at all.

To answer question No. 2, such technique would reduce engine wear only in that it would involve applying reverse thrust later in the landing roll and for a shorter time (assuming reverse thrust is disengaged always at the same speed, which is normally in the region of 60 kt), something that the pilot in the video might not have taken full advantage of.
Whether landing a 747 that way is a good idea, is a different matter, and Bianfable's answer quoting from the FCTM provides some indications.
Bonus info: regardless of the wisdom of doing wheelies on landing, the idea of delaying as much as possible the application of wheel brakes and reverse thrust (while dissipating as much kinetic energy as possible through 'free' aerodynamic drag) is sound.
The A380, for example, was the first to introduce the Brake To Vacate concept for the autobrake.
Unlike the ordinary autobrake settings LO, 2, 3, HI (which are still available and engage wheel braking at constant decelerations as soon as the landing gear is on the ground and the spoilers deployed), BTV starts applying brake pressure just in time to bring the aircraft to taxiing speed when it reaches the selected runway turn-off, not before.


text:
Can a non-IFR rated pilot accept an IFR clearance, provided the operation occurs in VMC only?
According to this FAA ruling a pilot can operate an aircraft under an IFR flight plan even if the aircraft is not IFR certified, provided the operation occurs in VMC.

What the ruling doesn't differentiate is whether the pilot doing that operation must be an IFR rated pilot (the ruling is with respect to instrument training, which implies there will be a properly rated CFII onboard).
I can't find the regulation that states that you must have an IFR rating to accept an IFR clearance/operate on an IFR flight plan. So, my questions are:

What regulation (if there is one) requires a pilot to have an IFR rating in order to operate an aircraft on an IFR flight plan?
Would a "VMC only" modifier change this requirement?

Operating as PIC under IFR always requires an instrument rating and currency, regardless of flight conditions.
14 CFR 61.3(e) requires the rating, and there's no exception for VFR conditions:

(e) Instrument rating. No person may act as pilot in command of a
civil aircraft under IFR or in weather conditions less than the
minimums prescribed for VFR flight unless that person holds:
(1) The appropriate aircraft category, class, type (if required), and
instrument rating on that person's pilot certificate for any airplane,
helicopter, or powered-lift being flown;

61.57(c) requires instrument currency, again regardless of flight conditions:

(c) Instrument experience. Except as provided in paragraph (e) of this
section, a person may act as pilot in command under IFR or weather
conditions less than the minimums prescribed for VFR only if:

See this question for more details on filing IFR flight plans without being instrument rated (not a good idea).
Michael Hall suggested in a comment that some pilots might think that to operate under IFR they only need to file a flight plan and get a clearance, because of the wording in 91.173:

No person may operate an aircraft in controlled airspace under IFR
unless that person has—
(a) Filed an IFR flight plan; and
(b) Received an appropriate ATC clearance.

However, 91.173 doesn't override or invalidate any other regulation. The general rule is that regulations add up and you have to follow all of them unless the wording makes it clear that something doesn't apply (e.g. 91.1(e) says that part 91 regulations don't apply to part 103 ultralights).

Good answer.  I was typing mine and didn’t notice that you had responded.  (Just so you didn’t think I was trying to “compete” for upvotes with the same quote!)  I will delete my answer because it is a dupe and you got there first, but do you think it might be worth mentioning 91.173?  Because I think that is the source of people thinking it’s OK to do...

@MichaelHall No problem :-) There's only one regulation, but there's more than one way to explain it! As for 91.173, that's a great point and I'll add something about that.


text:
Could ducted propellers improve efficiency in small drones?
I have seen that a new electric airplane design has decided to a choose ducted propeller design somewhat similar to the one of jet engines, explaining this design choice with the fact the ducted design would avoid loss of thrust from edges of propellers channeling thrust more efficiently.

I don't know how this could be calculated, but it's obvious this structure will add weight, and I'm wondering how it can be determined whether the gain in thrust efficiency compensates for the added weight and whether it makes sense to use a similar design on small drones.

Related: WP: Ducted fan.

If the ducted propeller really increases efficiency, we would see more of these

Rhein Flugzeugbau Fanliner (picture source)
and fewer of these:

Socata TB-9 (picture source)
The ducting has benefits when

the disk loading is very high. This means the propeller diameter is small for a reason. Since efficiency tanks with decreasing propeller size, another consideration forces the propeller to be small. This would be the tip Mach number.
the propeller noise would be unbearable without the fairing. Again, this is the realm of highly loaded propellers with high tip Mach numbers.
the flow passing through the propeller needs to be conditioned such that regardless of flight speed the flow speed at the disk is almost constant.

In short, this benefits turbofans which fly at transsonic speed.
For slower vehicles with less power hungry propulsion systems, the disadvantages will prevail:

Friction losses on both surfaces of the fairing easily eat up any gain from reduced tip losses.
The weight of the fairing creates induced drag.
Also, the fairing costs more to build and maintain than leaving it off and using a larger, more efficient propeller.

Could ducted propellers improve efficiency in small drones?

Clearly no!

I'm wondering how it can be determined whether the gain in thrust efficiency compensates for the added weight and whether it makes sense to use a similar design on small drones.

The authors of XFOIL, Mark Drela and Hal "Guppy" Youngren, have published and open-sourced a design code for ducted propellers. This article describes the process. For a good overview of other computational methods I recommend chapter 3 of this report. In short, you need a panel code to capture the complex interaction between duct and propeller blades.
In a very simplified form, a ducted propeller is equivalent to a slightly larger unducted propeller due to the reduction of tip losses. However, with increasing speed the added surface of the duct will add a drag component that is absent from the unducted propeller. In off-design conditions, the duct helps to normalize the flow through the propeller but becomes a liability with flow separation in the duct at higher angles of attack to the oncoming flow. There is no simple factor between ducted and unducted propellers and which is better depends on the operating conditions and the design preferences of each individual case.

Another benefit of the ducting is if you want to create a media buzz and lure gullible investors to support your hare-brained (or criminal?) scheme for a personal transport.

Ducted propellers were the same in the past as electric commuter planes are now. Investor baits.

@PeterKämpf you mean this: (unfortunately german) https://www.aerokurier.de/elektroflug/lilium-jet-dossier/ ?

Thanks for the clarification, but still remain obscure how the ducting gain should be calculated, so that numbers can be drawn to demonstrate when a configuration is not achieving any significant benefits from ducting or it's disadvantageous. The fact the Lilium data that doesn't add app is interesting, but still, it doesn't seem to mention anything on ducting vs non ducting matter. I mean the fact that they have inflated the data neither deny nor confirms if their choice of using ducted propulsion is advantageous or not compared to having open propellers.

@U_flow Yes, of course. You do so, too. It is obviously hare-brained. Same with the Cargolifter three decades back. That one started with honest intentions and was then hijacked by criminal New Economy MBAs. Interestingly, you linked to one of the non-fawning articles about Lilium. Well done!

It is actually suprisingly easy to give a lower boundary and therefore an estimate of the power consumption in hover for any thruster. It is given by momentum theory which correlates the size of an idealized rotor disk with its power consumption:
$$P = \sqrt{\frac{T^3}{2 \rho A}}$$
where:

T is the thrust
$\rho$ is the density of air (or other medium)
A is the area of the rotor disc (or diameter of thrust tube)
P is power

You can directly see that the bigger the rotor disc $A$, the lower the power consumption during hover.
Now, this is idealized in the sense that an ideal propeller accelerating air, would need this much power. Real propeller need more power, because of real world effects such as losses at the tip of the propeller or not-ideal drivetrain, mach effects and so forth and so on. These effects can be captured by correction terms such as $\eta_{tip}$ or $\eta_{drivetrain}$ ($\leq1$). Therefore the equation becomes.
$$P = \sqrt{\frac{T^3}{2 \rho A}} \cdot \frac{1}{\eta_{tip} \cdot \eta_{drivetrain}}$$
Therefore back to your question: All you do by adding a ducted fan, is to increase the efficiency of your propeller. Thus, for example $\eta_{tip}$ will rise perhaps from 0.8 to 0.95 or so (values are given only as an example).
But the ducts you depicted serve two purposes:

They maximize tip efficiency, therefore decreasing power consumption
They increase the diameter $A$ by "spreading out" the flow of air (for example with the big lip you see at the front of the duct), therefore further decreasing power consumption

Still, small propellers mean insane amounts of power. So much so, that there even was a discussion if for example this concept would even work or if it was fraudulant (unfortunately in german). I strongly encourage you to plug in some values for your favourite upcoming E-VTOL.
However, note the following: Helicopter have big rotors exactly to minimize power consumption during hover. EVTOL concepts plan to spend only very little time in hover, so for some cases it makes sense to take the efficiency hit of higher power consumption during hover in order to avoid having a big rotor during the high-speed portion of flight, because this also comes with a performance penalty.

Thanks for the explaination but remain the problem of the calc of η parameters you have mentioned, Knowing the way to calculate them it would be possible to determine the efficency gain of ducted solution and so know when the gain compensate the added weight and whether the added design complication is worth it

Also, the power/mass ratio of a combustion engine is a big part of the aircrafts mass budget, right? But electric motors are lighter (and outweighed by the batteries) so you could see a motor with lots of reserve power that's just used to take off?

@AndreaF: Unfortunately there is no "easy" way to calculate $\eta$, this is normally done empirically. To make things worse, it also depens how you construct your duct. If you do it wrong, the efficiency might even go down (see this post https://aviation.stackexchange.com/questions/88170/electric-ducted-fan-with-thrust-tube-thrust-and-exit-velocity-calculation/88225#88225)


text:
When is a high pressure system associated with IFR/MVFR conditions?
So I've read that typically high pressure systems bring good weather and clear skies. However, for the current high pressure system we're experiencing in central USA, we're having quite a bit of IFR/MVFR conditions.

The IFR/MVFR conditions down along the Gulf of Mexico where there's a stationary front make sense, but I'm curious why there's IFR/MVFR weather from Missouri through Illinois, Indiana, and Ohio in the midst of this high pressure system.
[
Is my understanding flawed about high pressure systems? Perhaps I should really be thinking of high pressure systems as stable air masses (in this case stable, low clouds). Or are there other factors at play leading to IFR/MVFR conditions being associated with this high pressure system?
In case it's relevant (and the time/date is difficult to read on the screenshots), the conditions described were happening around 6:00Z on 2019-01-02.

In the temperate zone that covers the US and Canada, weather systems are generally related to frontal boundaries between warm/wet and cold/dry air masses, where the cold air pushes under the warm air and forces it up.
But in an area with a large weak high pressure system, you can get a lot of weather caused just by solar convection, depending on the humidity and lapse rate of the air in the mass.  The weather systems will be kind of randomly distributed around, instead of being arrayed along a frontal boundary.
You can see it in the chart you posted.  The weather along the Gulf coast is probably related to the stationary frontal boundary that is there, whereas the weather in the central states is from local convective conditions.
The absolute best practical book on aviation weather theory I've ever read is called the RCAF Weather Manual, written by the Royal Canadian Air Force as its primary weather training guide for pilots.  It is so clearly and logically presented I cannot recommend it enough.


text:
What kind of malfunction is an "artificial horizon"?
I was once (like ten years ago) on a transpacific flight with Northwest from KIX/RJBB to KDTW on a Boeing 747 and our flight as delayed at the gate for about an hour because of some issue with a "floating horizon" (I remembered incorrectly, the issue was a "artificial horizon").
Is that a malfunction with the attitude indicator? What exactly went wrong? It sounded like they fixed it by swapping out a component, maybe the instrument itself?

Perhaps they wanted to say "an issue with the artificial horizon" (being the attitude indicator) and it got garbled in a game of telephone? Or was it a PA announcement from the flight deck?

"Artificial horizon" was the correct term. After a decade I just remembered the wrong term.

Best bet is that there was an issue with the artificial horizon. If it was on an older aircraft without a glass cockpit, the gyro or its gimbal was probably broken causing the face to 'float' around or just stick at an odd angle. Then it was then communicated poorly to you by someone. Other than that I have never heard of a "floating horizon" and a google search doesn't yield any results either.

I think you're right. Now that you've mentioned it, that sounds like the term they used. It had been so long, I think I remembered it wrong.


text:
What is the maximum age limit for getting a pilot licence?
Can I do pilot training courses even after age 26 ?

Please consider specifying which country and/or legislation you are interested in, answers might vary and making a comprehensive list for all world's country might be too much for the SE format.

Please also provide more information on what type of license? ATPL / CPL / PPL / SPL ...

As far as I know there are no maximum age restrictions on getting a pilots license anywhere in the world. Many countries have a minimum age restriction for getting a license, that usually does not prevent pilot training from taking place. 26 is fine, in fact 76 is fine as long as you pass whatever medical is required.

there are many glider pilots over 80

It should be noted that it may be more difficult for older pilots to pass the medical exam

Absolutely, that's probably one reason that there is no upper age limit. The system relies on the medical exam to ensure pilots are fit to fly.

To expand a bit on @GdD's answer, a number of aeronautical regulatory bodies, including the FAA, do not specify a maximum age for flight training or executing the privileges of Pilot-In-Command under most circumstances; as long as you hold a valid medical, you're in the clear.
That said, your individual circumstances may be different. There are some exceptions to this even in the civilian world, such as maximum age limits when flying for certain airlines (such as the age 65 rule for Part 121 operations in the US). If you're in a part of the world where airlines offer cadet programs and such, they may also impose maximum age limits for applicants.
If you're interested in military flying, the armed services may also have additional restrictions on maximum age for flight training; the USAF for example, had the age of 26 as the upper bound for anyone applying for a pilot slot about 5-6 years ago, and armed forces the world over have similar restrictions in place.
Edit:
Thanks to @SSumner for providing a link with the current age limits for US military pilot training.

For the US military you can see these ages - I think it's actually been longer than 5-6 years since the AF raised the limit to 28.5. And there are waivers for this - I went through with a guy who was about 34, and I know of at least one who was about 38 going through pilot training

Thanks for the reference. I'd heard about the age 26 limit from a former flight instructor of mine who was coming up on it and she was really happy that she was able to get her slot in time to begin UPT before that happened. Then again, she did get her pilot slot thru the Alabama ANG, so who knows?

It's certainly possible that  a guard unit could set a lower age for some reason. Active duty and guard can be very different in some areas

It would help if you specify if you are getting the license to fly private or commercially. For private there is no age restriction if you pass your medical, as is the same for commercial. However as mentioned some airlines may prevent you from flying after a certain time. That all being said this video brings up some good points about starting training later in life (in your 30's). It takes a lot of hours to become a commercial pilot and the later you start training the later you enter the workforce (as with any job). I would think 26 is perfectly fine. Im getting my Private Pilot and started at 23. There are plenty of people at the flight school well over 50 starting their training as well.